Many abdominal exercise apparatus/machines are known, the majority of which are confined to the “sit-up” or “crunch” movement where an individual, lying on one's back, flexes the trunk against the combined loads of upper body weight and gravity. By increasing the number of repetitive sit-ups endurance is improved over time and the abdominal muscles can become toned (more defined). However, there are no strength gains other than for the specific load value provided by upper body weight and gravity.
The known abdominal exercisers for home use are focussed on performing a “sit-up” or “crunch” movement in a consistent manner without contributing any load (adjustable or otherwise) of its own.
It is well established in physiological literature that in order to develop strength, muscles must be progressively loaded so that a maximum of 8 to 20 repetitions are performed.
Exercise machines that achieve this progressive loading are generally only available in a gymnasium because they tend to be large and heavy, often bolted to the floor. A typical device of this type requires the user to be sitting with the weight situated behind and pulled via an overhead pulley wheel into a bent forward (loaded) position (keeping one's back straight). The user then returns to the generally unloaded sitting position and repeats. This type of exercise is most closely related to a “sit-up” rather than “crunch” movement.
These machines are expensive and therefore not accessible to most people, particularly those with an interest in exercising at home.
It should also be noted that, when in a supine position, the initial movement performed in a crunch or sit-up is much more difficult than at the end of the movement. Available machines (as with weights in a gymnasium) apply a constant load. Users are limited by the initial load they must bear but may be capable of bearing greater loads toward the end of the exercise movement.
An exercise movement that has an increasing load as it is performed is thought to be more efficient for strength gains.